Method for operating electrolytic cells



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United States Patent US. Cl. 20495 9 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION Alkali metal chlorates may be produced by the electrolysis of aqueous solutions of alkali metal chlorides. The chlorine generated at the anode is reacted with the hydroxide ions generated at the cathode to produce the chlorates. Under ideal conditions, the production of sodium chlorate would consume all of the chlorine in the reaction:

6NaOH+3Cl NaClO -]-5NaCl-|-3H O This reaction can broadly be expressed as:

(31- +OH :OCl-- C As the concentration of hypochlorite decreases, conversion to chlorate is slower. Since hypochlorite should be recycled to the electrolytic cell only in low concentrations, conversion to chlorate or destruction of the hypochlorite is necessary. conventionally, the hypochlorite content remaining in a chlorate cell liquor is reduced via its decomposition in a retention tank, or by application of heat.

An additional problem affecting the operation of diaphragm type chlorate cells resides in the gradual chemical corrosion of the graphite anodes conventionally employed in this type of electrolytic cell. During the electrolysis of the feed brine in a chlorate cell, some oxygen is produced in the anode chamber. The oxygen attacks the carbon of a graphite anode converting it to carbon dioxide. The consumption of current at the anode in the production of oxygen results in a decrease of anode current efficiency. Likewise, the anode surface exposed to the electrolyte is gradually diminished in size or consumed by oxygen.

It has recently been found that organic compounds may be added to the anolyte of a chlor-alkali cell to prevent oxidation of a graphite anode through the sacrificial oxidation of the organic compound. This discovery forms the basis for the subject matter of copending application Ser.

No. 687,496, filed Dec. 4, 1967, in the names of E. H. Cook, Jr., and M. P. Grotheer.

It is an object of the present invention to provide a method of reducing the hypochlorite concentration in chlorate cell liquors being fed to a crystallizer.

It is another object of this invention to reduce the'hypochlorite concentration in chlorate cell liquors by means of a process which produces a product that may be recycled into the cell without adverse effects.

It is another object of this invention to reduce the oxyice gen attack on the graphite anodes of an electrolytic cell by means of introducing into the anode compartments an organic chlorohydrin, a glycol and/ or a polyol which has been produced by the reaction of an olefinic compound with the hypochlorite present in the cell liquors.

BRIEF DESCRIPTION OF THE INVENTION In accordance with this invention, an effective method for hypochlorite removal from diaphragm chlorate cell liquors containing small amounts of the same, is to contact the solution with an olefinic compound to effect the reaction:

It is understood that a halohydrin in the presence of an alkaline reagent such as sodium hydroxide, may be converted to the corresponding glycol or polyol. Likewise, the formation of a specific isomer of the glycol, polyol or halohydrin forms no critical feature for the purposes of this invention.

The reaction between an olefinic compound with the hypochlorite present in chlorate cell liquors may be conducted batchwise in a holding tank, or by cocurrent or countercurrent flow through a column. This reaction proceeds rapidly at a temperature between room temperature and about degrees centigrade. The reaction between hypochlorite and an unsaturated compound may be continued until the hypochlorite content is reduced to zero or the reaction may be controlled to afford a recycle liquor containing an acceptably low hypochlorite content compatible with the specific operating conditions and apparatus.

Gaseous olefinically unsaturated compounds are preferred reactants of this invention because they are most easily handled. The gaseous unsaturated organic compound may be bubbled through the chlorate cell liquor by any applicable means. However, liquid olefinically unsaturated compounds are also applicable and they may be employed in conjunction with a mixing tank or they may be allowed to react with the hypochlorite by allowing them to rise through the chlorate cell liquor (based on their specific gravity). It is most advantageous to employ an unsaturated reactant which is soluble in the recycled cell liquor, but the product halohydrin, polyol or glycol may be removed as such if desired.

The unsaturated organic reactants employed in this invention are olefinically unsaturated hydrocarbons containing 28 carbon atoms. The straight and branched chain aliphatic unsaturated hydrocarbons are applicable as well as cyclo aliphatic hydrocarbons such as cyclopentene, cyclopentadiene, cyclohexene, cyclohexadiene and the like. Of the unsaturated organic reactants, the most preferred olefinic compounds are ethylene, propylene, 2- methyl propene, l-butene, 2-butene, 1,3-butadiene, pentadienes such as 1,3- or 1,4-pentadiene and the like.

The advantages of this process are two-fold. First, the concentration of hypochlorite is reduced to a level satisfactory for crystallization of chlorate without attack by hypochlorite on the metal of the crystallizer. Second, the products, glycols, polyols, and halohydrins remaining in the mother liquor may be cycled into the electrolytic cell where they perform a heretofore unknown function of reducting oxidative attack on the carbon or graphite anodes. The more easily oxidized glycols, polyols or halohydrins are sacrificially oxdized at the anode in preference to carbon.

The overall result of the practice of this invention" is:

(l) A small loss of efiiciency caused by the destruction of some hypochlorite,

(2) Metallic crystallizers may be used with a minimum of corrosive attack,

(3) Low graphite consumption because of protection of the graphite from oxidation by the sacrificial oxidation of a glycol, a polyol and/ or a halohydrin.

As an added feature of this invention, in mercury cell technology, it is conventional to dechlorinate depleted brine by adding an acid and applying a vacuum and/or blowing air through acidified depleted brine. Also, the suggestion has been previously made to dechlorinate depleted mercury cell brine by reaction with ethylene under acidic conditions to produce dichloroethane, assymetric tetrachloroethane and perchloroethylene.

The instant invention contemplates neutralizing a depleted mercury cell brine and reacting it with an unsaturated aliphatic hydrocarbon to remove the remaining hypochlorite. The halohydrin, glycol and/or polyol product is more desirable than a haloalkane or haloalkene of the prior art because of the improved benefit in the use of the halohydrin, glycol or polyol as an additive to the mercury cell feed brine. The halohydrin, glycol and/or polyol product, when added to mercury cells acts to reduce graphite consumption and in addition, the additives tend to reduce the hydrogen content in the chlorine produced in a mercury cell.

DETAILED DESCRIPTION OF THE INVENTION This invention may best be understood by reference to the accompanying drawing.

The drawing represents a fiow diagram of a typical process involving the use of a diaphragm chlorate cell in the production of an alkali metal chlorate.

Example 1 To demonstrate the retardation of graphite anode consumption through the addition of an organic compound, such as ethylene glycol, the following example was performed. Ethylene glycol is an example of a product which may be produced by the reaction of ethylene with hypochlorite present in chlorate cell liquors. This example represents an accelerated test.

Two electrolytic cells containing graphite anodes and cathodes were operated without diaphragms at a temperature of 60 degress centigrade at a current density of 1.5 amperes per square inch. The electrolyte contained 100 grams per liter sodium chloride and 200 grams per liter sodium chlorate. Ethylene glycol was added to one of the cells at a concentration of 1 gram per liter. The electrolytic solutions were discarded once a day and fresh electrolyte was added to each cell. The ethylene glycol was also replenished daily. The cells operated for a total of 100 hours.

As a result of this operation, the graphite anode used in the solution with no ethylene glycol was severely attacked. A layer of about inch thickness almost completely sloughed oif. There were also signs of microcracking caused by deep penetrating attack. The graphite from the solution with ethylene glycol additve also showed some attack but it was more uniform, leaving a compact anode that showed no signs of micro-cracking or delamination. Thickness measurements showed a 19.4 percent decrease for the anode in the solution without ethylene glycol compared to a 8.1 percent decrease for the anode exposed to an ethylene glycol-containing solution.

Similar results are obtained when the halohydrins and/ or polyols produced by the reaction of the olefinically unsaturated compounds described supra with hypochlorite in the cell liquor, are returned to the cell with made-up feed brine.

Example 2 The depleted brine from a mercury type chlor-alkali cell was subjected to a light vacuum to partially remove dissolved chlorine. The resulting brine was treated to afford a neutral solution. Then, the solution was contacted with ethylene at about 70 degress centigrade to produce the corresponding chlorohydrin and/ or glycol with partial or complete removal of hypochlorite. To this chlorohydrin-containing solution, sodium chloride was added. The solution was recycled to the mercury cell for electrolysis with a resultant reduction in graphite anode consumption and diminished quantities of hydrogen in the chlorine gas produced in the cell.

Having disclosed this invention, it is obvious to those of average skill in this art that various modifications of this invention may be made which do not depart in spirit from the true scope of this contribution.

What is claimed is:

1. A process for removing hypochlorite from an electrolytic cell liquor which is to be recycled to an electrolytic cell equipped with a carbon anode which comprises the steps of:

(a) contacting said cell liquor with an olefinically unsaturated compound at a temperature between room temperature and about 105 C. to produce a reaction product of said hypochlorite and olefin selected from the group consisting of a chlorohydrin, a glycol, a polyol, and mixtures thereof;

(b) recycling said cell liquor containing said reaction product with made-up feed brine to said electrolytic cell;

(c) electrolyzing the feed brine of step (b) in said electrolytic cell.

2. The process of claim 1 in which said olefinically unsaturated compound is an aliphatic or cycloaliphatic hydrocarbon of 2-8 carbon atoms.

3. The process of claim 1 in which said unsaturated compound is a member of the group consisting of ethylene, propylene, 2-methylpropene, l-butene, 2-butene, 1,3-butadiene, 1,3-pentadiene, 1,4-pentadiene and mixtures thereof.

4. The process of claim 1 in which said electrolytic cell is a diaphragm type chlorate cell.

5. A process for reducing oxidation of carbon anode in a diaphragm type electrolytic chlorate cell which comprises reacting the chlorate cell liquor containing residual amounts of hypochlorite with an olefinically unsaturated hyrdocarbon compound to produce a product selected from the group consisting of a chlorohydrin, a glycol, a polyol, and mixtures thereof, passing the treated solution to a crystallizer, and recycling the mother liquor containing the products of reaction between said hypochlorite and said hydrocarbon compound plus make up brine to the anode compartment of said diaphragm type electrolytic chlorate cell.

6. The process of claim 5 in which said unsaturated compound is an aliphatic or cycloaliphatic hydrocarbon of 2-8 carbon atoms.

7. The process of claim 5 in which said unsaturated compound is a member of the group consisting of ethylene, propylene, Z-methyl-propene, l-butene, Z-butene, 1,3-butadiene, 1,3-pentadiene, 1,4-pentadiene and mixtures thereof.

8. A process for removing hypochlorite from the depleted brine of a mercury cell containing a carbon anode comprising bringing the depleted brine to near neutrality by treatment with a base, then contacting said depleted brine with an unsaturated hydrocarbon compound to produce a product selected from the group consisting of a chlorohydrin, a glycol, a polyol and mixtures thereof, reconstituting said depleted brine with alkali metal chloride to afford an acceptable mercury cell feed brine, recycling said reconstituted feed brine to said mercury cell for electrolysis, said chlorohydrin, glycol, polyol and mixtures thereof serving to decrease carbon anode oxidation and reduce the amount of hydrogen evolved with chlorine in the mercury cell.

9. The process of claim 8 in which the depleted mer cury cell brine is partially dechlorinated prior to treatment with a base.

References Cited UNITED STATES PATENTS 6 1/ 1918 McElroy 20480 8/ 1960 Neipert 204-99 TA-HSUNG TUNG, Primary Examiner US. Cl. X.R. 

